1. Field of the Invention
The present invention relates to a technique of detecting an electrical angle of a synchronous motor in a sensor-less manner as well as a technique of controlling the synchronous motor. More specifically the present invention pertains to these techniques when the synchronous motor is either at a stop or at a low-speed rotation.
2. Description of the Related Art
In a synchronous motor that rotates a rotor through an interaction between a magnetic field occurring when multi-phase alternating currents are flown through windings and a magnetic field produced by permanent magnets, in order to obtain a desired rotational torque, it is required to control the multi-phase alternating currents according to the electrical angle or the electrical position of the rotor. The electrical angle may be detected with a sensor, such as a Hall element. It is, however, desirable to detect the electrical angle in a sensor-less manner, with the view to assuring the reliability of a control apparatus of the synchronous motor.
There is a known method that detects the electrical angle in a sensor-less manner when the synchronous motor is either at a stop or a low-speed rotation. This known method takes advantage of the phenomenon that the magnetic resistance in a magnetic circuit varies with a variation in angle of the rotor and changes the inductances of the windings in a salient-pole permanent magnets-type motor.
The following description regards a permanent magnets-type three-phase synchronous motor. The synchronous motor is expressed by an equivalent circuit that has three-phase coils of U, V, and W phases and a permanent magnet rotating about an axis of rotation as shown in FIG. 4. In the equivalent circuit, the axis that passes through the N pole of the permanent magnet as the positive direction is defined as a d axis, whereas the axis that is electrically perpendicular to the d axis is defined as a q axis. The magnetic field in the q-axis direction is a main dominant factor of the torque of the motor. The electrical angle is defined as a rotational angle .theta. of the axis passing through the U-phase coil and the d axis. Ld denotes an inductance of the windings when a voltage is applied to cause a magnetic field in the d-axis direction, and Lq denotes an inductance of the windings when a voltage is applied to cause a magnetic field in the q-axis direction. A motor control apparatus, which controls operation of the synchronous motor, can not first detect the position of the rotor accurately. The motor control apparatus accordingly estimates an electrical angle .theta.c, which causes an angular error .DELTA..theta. deviated from the true electrical angle .theta. as shown in FIG. 4. The axes estimated as the d axis and the q axis based on the electrical angle .theta.c by the motor control apparatus are respectively referred to as a .gamma. axis and a .delta. axis.
In order to detect the electrical angle in this state, the motor control apparatus applies a voltage in the .gamma.-axis direction and detects electric currents flowing in the .gamma.-axis direction and in the .delta.-axis direction corresponding to the applied voltage. In the case where the .gamma. axis estimated by the motor control apparatus coincides with the d axis, no electric current is detected in the .delta.-axis direction. When there is an angular error .DELTA..theta. of the electrical angle, on the other hand, an electric current is detected in the .delta.-axis direction. The electric currents in the .gamma.-axis direction and in the .delta.-axis direction vary with a variation in angular error .DELTA..theta.. This accordingly enables calculation of the angular error .DELTA..theta. based on the observed electric currents and detection of the electrical angle .theta.. The method of calculating the angular error .DELTA..theta. will be discussed later.
This known method enables the electrical angle to be detected with a relatively high accuracy when the synchronous motor is either at a stop or a low-speed rotation. In the synchronous motor under such driving conditions, however, this method is not applicable to detect the electrical angle when the torque command value increases to or above the rated torque of the motor. The studies on the technique of controlling the motor in a sensor-less manner have only recently started and not yet referred to this problem.
One possible countermeasure designs the motor to have a sufficiently marginal rating relative to the required torque. This arrangement, however, causes another problem of increasing the size and the weight of the motor.